FIXED (SLOW-MOVING) BED UPDRAFT GASIFICATION OF AGRICULTURAL RESIDUES

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1 FIXED (SLOW-MOVING) BED UPDRAFT GASIFICATION OF AGRICULTURAL RESIDUES Alejandro Grimm, Emilia Björnbom and Rolando Zanzi Dept. of Chemical Engineering and Technology / Divison of Chemical Technology Royal Institute of Technology (KTH), S-1 Stockholm, Sweden ABSTRACT: A laboratory-scale countercurrent fixed-bed gasifier has been designed and constructed to produce data for process modelling and to compare the gasification characteristics of several biomasses. Densified woody biomass, birch, in form of pellets with a diameter of mm and a length between 5 and 15 mm has been used as a raw material for batch autothermal gasification using air as an oxidation agent. The main objectives were to study the effect of the treatment conditions on the distribution of the products and the composition of product gas to establish the suitability of the gasifier to produce combustible gas with sufficiently high calorific value. The influence of the air flow rates on the composition of the producer gas has been studied. The amount of the biomass used in the experiments was varied between 1 and kg and the flow rate of the oxidation agent, air, was varied from 1,1 to, m 3 /h. Keywords: gasification, fixed bed, pellets. 1 INTRODUCTION Gasification is a widely studied and applied technology to produce a mixture of combustible gases. It consists of several sequential processes which include: drying, pyrolysis to give gases, tars and char, cracking and oxidation of tars and partial oxidation of pyrolysis gases and gasification of char. Complex physical, chemical, mass- and transport phenomena take place in the gasifier. Fixed bed reactors are used in small-scale gasification while large biomass gasifiers are usually of fluidized-bed or entrained flow type. Fixed bed, updraft and downdraft reactors are, in general, of very simple construction and operation, and avoid the excessive costs of feedstock pulverization [1]. The downdraft gasifier features concurrent flow of gases through a descending packed bed. This configuration results in a high conversion of pyrolysis intermediates and hence a relatively clean gas. The physical limitation of the diameter and particle size relation means that there is a practical upper limit to the capacity of this configuration of 5 kg/h or 5 kw E. In the updraft gasifier the downward-moving biomass is first dried by the upflowing hot product gas. After drying, the solid fuel is pyrolysed, giving char which continues to move down to be gasified, and pyrolysis vapours which are carried upward by the upflowing hot product gas. The tars in the vapour either condense on the cool descending fuel or are carried out of the reactor with the product gas, contributing to its high tar content []. The product gas from an updraft gasifier thus contains a significant proportion of tars and hydrocarbons, which contribute to its high heating value. Usually the gases are directly used in a closely coupled furnace or boiler. The fuel gas requires substantial cleanup if further remote processing is to be performed. There is an interest in cleaning of the updraft gas for electricity production, as low temperature tars are more reactive and thus easier to be removed, than the hightemperature tars produced in much lower amounts by downdraft and fluidized bed gasifiers [3]. The principal advantages of updraft gasifiers are their simple construction and high thermal efficiency: the heat of the gas produced is recovered by direct heat exchange with the entering feed, which thus is dried, preheated and pyrolysed before entering the gasification zone. Updraft gasifiers are able to gasify very wet fuels. Updraft gasifiers are suitable for sizes between and MWe. Several industrial updraft gasifiers are in operation in Northern Europe for peat as well as straw and wood chips. A large effort is put into the development of small scale combined heat and power plants based on updraft biomass gasification combined with an internal combustion engine. Significant part of the work has been related to the cleaning of the product gas intended for use in gas engines and reliable solutions based on gas cooling, wet electrostatic precipitation and novel technologies for cleaning have been demonstrated []. Recently it has been shown that product gas from an updraft gasifier containing tar and particles could also be used directly in a Stirling engine without further cleaning [5]. In this work, wood pellets with a diameter of mm. and a length of 1-3 mm, have been used as a raw material. Air was applied as an oxidation agent. The effect of the treatment conditions on the temperature profile in the reactor and the distribution of gaseous products were of primary interest. EXPERIMENTAL.1 Updraft gasifier at KTH Figure 1 shows the updraft gasifier designed at KTH. It consists of a tube with an internal diameter of.11 m, mm thickness and.5 m height. The gasifier is insulated externally with a cm. thick refractory material (glass wool). The grate, which supports the biomass and also serves as an air distributor is 3 mm thick with grooves of mm diameter. The temperature profile is followed by a set of three thermocouples situated at different heights in the gasifier; temperature T 1 is measured at 3 mm, T at 35 mm, and T 3 at 51 mm distances measured from the grate of the gasifier. The expected capacity of the gasifier is 1-15 kwth.

2 Figure 1. Updraft gasifier designed at KTH : (1) feeder, () updraft gasifier, (3) thermocouples, () viewer, (5) distributor, () inlet air, (7) cone (), ash valve, (9) closed water tank (gas scrubbing), (1) tar and char collector, (11) cyclonic condenser, () rotameter, (13) packed-bed filter. The first step in the gasification experiments was ignition of the fuel bed. Initially.3 Kg of charcoal was burned over the air distributor, to rise the temperature to about 5 C. Then the selected for the experiment total amount of the biomass was fed into the reactor through a feeder. Different air factors, defined as ratio of actual air flow rate to the stoichiometric airflow rate [], could be achieved adjusting the airflow rate. Cyclone condenser, wet scrubbing, acetone containing flasks and absorption column constitute the cleaning and cooling system. The cyclone condenser captures part of the existing tar and the solid particles, which follow with the gas. The liquid phase is discharged from the bottom of this cyclone and collected for being measured. Wet scrubbing takes place in a glass column cooled by water. In this step, most of the tar is separated from the gas. Three flasks containing acetone are used to clean the rest of tars and to cool the gases to ambient temperature before sampling. The last cleaning unit is a cylindrical glass pipe filled up with cotton wool and phosphorus pentoxide, which acts as a desiccant, drying the product gas. The cleaned gas is analysed in a chromatograph for,,, N,, saturated hydrocarbons (C1-C5) and aromatic hydrocarbons (benzene and toluene).. Raw material The biomass used in the experiments is wood pellets. The utilization of biomass in form of pellets and briquettes has recently gained significant interest [7]. Pelletisation is a way of improving the fuel handling, transportation, conversion, and storage. Pelletisation causes a disorder in the anisotropic structure of biomass thus the fuel pellets are much more uniform compared to the initial biomass. Requirements for standardization in production of pellets have been introduced in several countries in Europe. The most strict standard requirements are applied in Austria []. Utilization of densified biomass in form of pellets has contributed significantly to the recent progress in wood combustion. Using wood pellets in improved combustion installations and better control of the temperature and the excess air ratio have resulted in combustion units with high efficiency and low emissions [,9]. Pelletisation of voluminous non-uniform agricultural and forest residues increases the energy density of these materials and improves their fuel properties. Wood wastes from sawmills or bagasse from sugar industries are usually combusted to satisfy the local heat and energy demands. They may, however[1,11]. The largely available wastes from the seasonal sugar cane industry may be densified into pellets to facilitate the storage and the all-year utilization for electricity production via gasification. Biomass pellets have, however, scarcely been studied for gasification []. Wood pellets are already commercialized and the utilization in heat power plants as well as for residential heating has increased significantly during the recent years. 3 RESULTS The experiments were performed using single feeding of the biomass in the reactor (batch experiments), thus the duration of the gasification was limited. The amount of the biomass used for the experiments and the flow rate of air have strong effect on the results. The main gaseous products were carbon monoxide and carbon dioxide due to both combustion and gasification. Small amounts of hidrogen, methane and C hydrocarbons were also produced. Experiments using Kg wood pellets were shown in Figure. The curves shows the temperature of the combustion zone, T 1 and the concentration of gaseous products at different flow rate of air used,, 1.5 and 1.1 m 3 /h. The quick consumption of biomass in combustion at high flow rate of air causes fast decrease in the concentrations of gaseous products (Figure a). At lower flow rates of air (Figure b) the time needed to heat up the reactor and to obtain maximum concentrations of gaseous products is longer, the consumption of biomass is also slower and the curves illustrating the concentrations of gaseous products show slower decrease. Highest concentrations of gaseous products using kg of biomass are obtained using a flow rate of air m 3 /h. Carbon monoxide and dioxide dominate in the gaseous product. The concentrations of other gaseous products such as hydrogen, methane and particularly C - hydrocarbons are much lower. Experiments using 3 and Kg wood pellets were performed. The flow rate of the oxidation agent (air) was varied as it follows:.,, 1.5 and 1.1 m 3 /h. The initial effect of increased air flow rates was higher temperatures due to increase of the released heat in the combustion of biomass and thus improved conditions for the gasification. However, at higher air flow rates in the batch experiments performed in this

3 study, the amount of the biomass available in the gasifier, was quickly consumed. Thus the duration of the gasification was rather short. This can clearly be seen in the figures 3,, 5 and. CHx a CHx b Figure. Concentration of gaseous products and temperature profile; kg wood pellets. Air flow rate: a) m 3 /h, b) 1.1 m 3 /h. Figures - compare the temperature in the fuel bed T 1 and the concentrations of the gaseous products obtained using 3 and kg biomass and.,, 1.5 and 1.1 m 3 /h flow rates of the air. The use of higher air flow rate,. and m 3 /h resulted in development of higher temperatures in the gasifier, approximately 1 o C, and higher concentrations of the gasification products: carbon monoxide (1- mol %) hydrogen (- mol %) and methane (-5 mol %) in (Figures 3 and ). The consumption of biomass at higher air flow rates is, however, fast and favourable conditions for gasification were only during a short interval of time. In the experiments using 3 kg biomass and an airflow rate of m 3 /h, hhe curves showing the concentration of the gaseous products, H and CH show maximums at approximately min from the beginning of the experiments. 1 1 CHx CHx 3a 3b Figure 3. Concentration of gaseous products and temperature profile; a) kg and b) 3 kg wood pellets. Air flow rate: m 3 /h. Lower air flow rate, 1,1 m 3 /h, developed lower temperatures in the gasifier, approximately C and lower maximum concentration of the gasification products: (-15 mol %), H (3- mol %) and CH (-3 mol %) (see the maximums in the curves in Figure 7 using 3 and kg biomass). As expected, the experiments performed with lower air flow rate, 1,1 m 3 /h (Figure 5, a and b) do not show the fast consumption of biomass observed at the higher air flow rate (Figure 3 and ) and a high cncentration of gaseous products is obtained suring a longer time (Figure 5a).

4 1 1 CHx a CHx a CHx b CHn b Figure. Concentration of gaseous products and temperature profile; a) kg and b) 3 kg wood pellets. Air flow rate: 1,5 m 3 /h. Figure 5. Concentration of gaseous products and temperature profile; a) kg and b) 3 kg wood pellets. Air flow rate: 1,1 m 3 /h. Amount of biomass less than 3 kg was not sufficient for auto thermal gasification in the reactor used in this study. Smaller amounts of feed (1 to kg) and lower airflow rates resulted in lower temperatures in the reactor and low heating values of the gaseous products. The conditions for gasification and the calorific value of the gaseous product were improved, using larger amounts of biomass and higher airflow rates. The heating value of the gaseous product obtained from 3 kg pellets using low airflow rates (1,1 to 1,5 m 3 /h) was approximately,5 MJ/ m 3. The heating value increased to 3-3,5 MJ/ m 3 using bigger amount of feed ( kg. biomass and the same airflow rates). In the selected experimental conditions highest heating values of the gaseous products (-, MJ/m 3 ) were obtained using 3 to kg biomass feed and -, m 3 /h airflow rates. The results from these initial tests may be used to improve the construction of the gasifier and the operating conditions. 1 CHx Figure Figure. Concentration of gaseous products and temperature profile; kg wood pellets. Air flow rate:. m 3 /h

5 Liquids (ml) kg 3 kg 1 1,5,5 3 Air flow (m 3 /h) Figure 7 Figure 7. Liquid product (tar + water) obtained from 3 kg and kg wood pellets. Figure 7 shows the dependence of the volume of the liquid products on the air flow rate. The higher the rate the larger the amount of liquid products is. The large amount of tar obtained at air flow rate may be attributed to the short residence time of the tar in the reactor hampering the cracking reactions. NCLUSIONS The predominant effect of the increase of the air flow is higher temperatures because the amount of combusted biomass and heat released increase. The air/fuel ratio decreases with larger air flow. At higher air flow rate, the gasification process is favoured and the concentration of carbon monoxide increases at expenses of the carbon dioxide. If the air flow becomes higher than a certain limit, the temperature and the gasification rate decrease and the air/fuel ratio increases. In the selected experimental conditions the flow rate of the oxidation agent, the air, had strong effect both on the temperature profile in the gasifier and on the distribution of products. The higher the flow rate of air the shorter the time required to rich high temperatures and maximum concentrations of the gaseous products. High rates of gas flow favor formation of high concentrations of gaseous products and large amounts of tar. Carbon dioxide and carbon monoxide dominate among the gaseous products. The concentrations of hydrogen, methane and particularly C - hydrocarbons are much lower. The short residence time of the volatiles in the gasifier and the insufficient thermal cracking of the tar at high flow rate of the air contribute to increased tar yield. REFERENCES [1] A.D.J. Purnumo and K.W. Ragkand, Pressurized downdraft combustion of wood chips, 3th Symp. on Combustion, The Combustion Institute, pp. 5-13, 199, [] A.V.Bridgwater,, K. Maniatis and H.A. Masson. In Pyrolysis and Gasification (Eds G.L. Ferraro et al.), EUR 73, Commission of the European Communities, Luxembourg, pp. 1-5, 199. [3] A.G. Buekens, A.V. Bridgwater, G.L. Ferrero and K. Maniatis, Commercial and marketing aspects of gasifiers, EUR 73, Commission of the European Communities, Bruxelles, Belgium, 199. [] B. Teislev, Wood-chip updraft gasifier based combined heat and power, nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, 1-1 May Palazzo dei Congressi, Rome, Italy, Ed. by: Van Swaaij, Fjällström, Helm, Grassi, ISBN , ISBN , ETA-Florence and WIP-Munich, p ,. [5] N. Jensen, J. Werling,., H. Carlsen and U. Henriksen, CPH from updraft gasifier and stirling engine, th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection. Amsterdam, 7-79, [] C. Di Blasi, 1999, Countercurrent Fixed-Bed Gasification of Biomass at Laboratory Scale, Ind. Eng. Chem. Res., 3, pp , [7] I. Obernberger and G. Thek, Physical characterisation and chemical composition of densified biomass fuels with regard to their combustion behaviour, Biomass and Bioenergy 7, 53 9,. [] F. Fiedler, The state of the art of small-scale pelletbased heating sysytems and relevant regulations in Sweden, Austria and Germany, Renewable and Sustainable Energy Reviews, 1-1,. [9] T. Nussbaumer, Combustion and Co-combustion of Biomass: Fundamentals, Technologies and Primary Measures for Emission Reduction, Zurich, Energy and Fuels, 3. [1] T.M. Kayal, M. Chakravarty, Mathematical modelling of continuous updraft gasification of bundled jute sticksa low ash content of woody biomass. Biores. Technol. 9 1, 199. [11] Kurkela, E., P. Stahlberg, P. Simell and J. Leppalahti, Updraft gasification of peat and biomass, Biomass, 19,37, 199. [] C. Erlich, M. Öhman, E. Björnbom and T. Fransson, Thermochemical characteristics of sugar cane bagasse pellets, Fuel,, ACKNOWLEDGEMENTS The authors acknowledge the financial supports of the Swedish Agency for Research Cooperation with Developing Countries, Department for Research Cooperation (Sida-SAREC) and the European Community (ALFA Programme), project -FA.